Development of an integrated Total Internal Reflection (TIR) optical switch necessitates the creation of a region of light propagation with controllable change in refractive index. This region with refractive index change has an effective change in reflectivity to the incident light via the Fresnel relations.
The Fresnel relations describe the reflection and transmission of light as it is incident on an interface between two regions at some angle of incidence. The amount of light transmitted and reflected depends on both the angle of indigence and the difference in the refractive indices of the two regions. For a fixed difference in refractive index between the two layers there is some angle for which all of the incident light is reflected. This is called total internal reflection, and the incident angle producing it is called the critical angle. Similarly, for a fixed angle of incidence there is some refractive index difference that also creates total internal reflection. Also, if the is no difference in refractive index between the two regions, then the reflection is zero for all angles. By increasing the difference in the refractive indices between the two regions, the critical angle becomes closer to the normal to the interface. By making the critical angle closer to the normal to the interface, the physical area used by the switch and the associated waveguide ports is reduced. This means that the maximum controllable change in index of refraction is desirable to create the smallest integral TIR optical switch. The TIR optical switch uses a fixed angle of incidence between the input waveguide and the controlling electrode of the TIR optical switch, and then uses control of the refractive index difference between the waveguide region and the controlling electrode region to operate the device. Knowledge of the refractive index difference obtainable with the TIR optical switch allows the fixed incident angle to be chosen so that the total internal reflection of the incident light is obtained when the device is on.
Control of this change allows selection of the incident light to be transmitted (TIR off) to output port 1, or reflected (TIR on) to output port 2. Current investigation into creating this controllable change in refractive index for use in a TIR optical switch is centered in two major areas. One uses a forward biased p-n junction in which the light is incident laterally on the junction (see K. Ishida et al, InGaAsp/InP Optical Switches Using Carrier Induced Refractive Index Change, Applied Physics Letters, Volume 50(3), pp 141-142, 19 Jan. 1987). The other uses an electric field applied perpendicularly to a multiple quantum well region where the incident light is perpendicular to the applied field (see H. Yamamoto et al, Intersectional Waveguide Type Optical Switch With Quantum Well Structure, Transaction of the IECE of Japan, Volume E68, pp 737-739, 1985).
The forward biased p-n junction TIR switch is based on the change in refractive index due to the introduction of free carriers in the junction. The magnitude of change is limited only by the maximum forward bias current of the junction, i.e., the amount of charge in the junction. However, the forward biased p-n junction TIR switch is limited in speed by the recombination lifetimes of the electron-hole pairs, which is on the order of a few nanoseconds.
The electric field controlled TIR switch uses the refractive index change attributable to the Quantum Confined Stark Effet (QCSE) found at room temperature in quantum wells. (For example, see Weiner et al, Quadratic Electro-Optic Effect Due To The Quantum-Confined Stark Effect in Quantum Wells, Applied Physics Letters, Volume 50(13), pp 842-844, 30 Mar. 1987). The QCSE optical switch is based on the change in excitonic absorption within a quantum well due to an applied electric field. This change in absorption is coupled to a change in refractive index via the Kramers-Kronig relations. Typical device geometries use a reverse biased p-i-n structure, where the multiple quantum wells compose the intrinsic region. The switching capability of these devices is limited by the breakdown field of the material (approximately 10.sup.+5 V/cm). It is noted that the apparent breakdown field (i.e., when the destructive breakdown of the device occurs) is lowest in a p-i-n junction because the field creates a large depletion region in which many impact ionization collisions may occur. If the high field region were to be limited to a small volume, say a single quantum well, such that sufficient energy for ionization cannot be obtained, then a field much higher than the p-i-n junction breakdown may be applied without device breakdown.